Tagged unions support

[mariusmuja]

Is your feature request related to a problem? Please describe.

The following code uses tagged unions with Literal integer as discriminant:

from typing import Tuple, Union, Literal

MsgA = Tuple[Literal[1], str]
MsgB = Tuple[Literal[2], float]

Msg = Union[MsgA, MsgB]

def handle_a(a: MsgA):
    pass

def handle_b(b: MsgB):
    pass


def handle(m: Msg):
    if m[0] == 1:
        handle_a(m)
    elif m[0] == 2:
        handle_b(m)

This type checks successfully with mypy as documented here (https://mypy.readthedocs.io/en/stable/literal_types.html#tagged-unions), however this type narrowing pattern is not supported by pyright.

Describe the solution you'd like

It would be great if pyright would also support this type narrowing pattern.

[erictraut]

This feature will be included in the next release of pyright and pylance.

[mariusmuja]

Thank you for adding this feature so quickly!

With this I can now remove lots of if TYPE_CHECKING: hacks from the code.

[erictraut]

This is addressed in pyright 1.1.161, which I just published. It will also be included in the next release of pylance.